1. Field of the Invention
This invention relates to surgical devices and is concerned with a prosthetic patella replacement for use as a surgical implant.
2. Description of Related Art
The surgical management of arthritis of the knee involves external supports such as bandages, braces and walking aids. Operative intervention may include anaesthetic, intra-articular steroid injection or radio-isotope synovectomy, arthroscopic washout, debridement, menisectomy, chondroplasty or synovectomy, esteotomy or an arthroplasty.
Joint arthroplasty may be an excision arthroplasty or interpositional or fascial arthroplasty or a prosthetic arthroplasty. Prosthetic arthroplasty of the knee may be undertaken as a uni-compartmental arthroplasty, a bi-compartmental arthroplasty, a patello-femoral arthroplasty or a total knee arthroplasty. The total knee arthroplasty may be subdivided into a hinged or linked arthroplasty although this has been superseded by the total condylar arthroplasty which may be unconstrained, semi-constrained or constrained according to the freedom of motion conferred upon the arthroplasty by its design (Goodfellow and O'Connor 1986).
Patello-Femoral replacement was not initially performed as a part of total knee arthroplasty (Sneppen et al 1978). The anterior surface of the femur was left uncovered articulating directly with the patella. This resulted in unacceptable knee pain and poor function. Therefore modifications were introduced which resurfaced the anterior surface of the femur (Insall et al 1976). This still resulted in unacceptable anterior knee pain and poor function. Therefore a patella prosthesis was developed which was initially a polyethylene dome shape fitting flush on top of the patella after the articular surface was resected (Moreland Thomas and Freeman 1979). A small keel or peg was fitted down in the cancellous bone of the patella to stabilise the prosthesis onto the bone. This patella dome was cemented in situ with acrylic cement (Install, Lachiewich and Burstein 1982, Wright et al 1990).
As un-cemented prostheses became popular in the early 1980's the design of the patella was changed in most implants (Hungerford and Krackow 1985). The basic concept was unaltered. However, a metal back was attached to the implant to reduce deformation of the polyethylene, to allow initial fixation without acrylic cement and to allow bone ingrowth into the porous metal backing of the patella thus giving long term stability within the bone (Kenna and Hungerford 1984). However the recent reports now emerging throughout the world refer to great problems with such a design. As the polyethylene thickness is reduced to allow for the metal backing, the polyethylene becomes liable to disintegrate under the stresses to which it is submitted. The wear rate progresses through the thin polyethylene at an earlier stage, the metal backing may then be exposed and cause squeaking or scratching of the implant and metallic debris may be added to the plentiful polyethylene debris produced. The manufacture of the metal backing has been difficult, leading to disruption of the pegs from its reverse side. The metal backing and the polyethylene may become dissociated, and manufacture of a harder carbon fibre impregnated polyethylene patella dome has led to excessive wear, release of carbon fibre into the knee and disruption of the polyethylene due to faulty manufacture. Patella fracture of dislocation may also occur and this may be more common with the thicker designs of patella implant or where insufficient patella bone is resected (Scott et al 1978, Mochizuki and Schurman 1979, Clayton and Thirupathi 1982, Ranawat 1986, Simison, Noble and Hardinge 1986, Lynch, Rorabeck and Bourne 1987).
The incidence of patello-femoral complications following knee arthroplasty is now the most common complication and has been reported as being up to 64% of cases (Sneppen et al 1978, Clayton and Thirupathi 1982, Merkow, Soudry and Insall 1985, Scuderi et al 1989).
Another factor which has received little attention is the shape of the articulating surface in relation to patello-femoral stability in the lateral plane. It is accepted that the anterior flange of the femoral component must be long enough to prevent lateral tracking or soft tissue impingement (Sneppen et al 1978, Thorpe, Bocell and Tullos 1990). Hungerford suggested an anatomical shape of patella button with the maximal thickness placed laterally. However the use of this implant fell into disrepute with the difficulties of insertion and high complication rate (Kenna and Hungerford 1984, Hungerford and Krackow 1985). Recently a design has been released with a central post and a surrounding shelf at a lower level. This was produced to aid patello-femoral stability by increasing the lateral resistance to patella subluxation from the patello-femoral groove. The incidence of patello-femoral complications has led many surgeons to abandon patella resurfacing (Scott et al 1978, Picetti, McGann and Welch 1990).
There has been little concern with the mechanics of the human patella in relationship to the articular geometry. Thus the patella is a torque converter between the pull vertically of the quadriceps muscle and the patella tendon acting on the tibial to extend the knee joint. The patella articulates at its thickest inferior pole in knee extension thus maintaining the point of articulation within the depth of the patella groove distally where it is more stable, but also shifting the quadriceps tendon anteriorly thus increasing the moment around the knee and generating a greater extension force on the tibia near full extension. This produces an advantageous mechanical advantage to the quadriceps and also confers greater stability to the patello-femoral joint.
The human patella during knee flexion articulates upon its medical and lateral edges, so allowing the patella to sink between the femoral condyles and a greater range of knee flexion for a given quadriceps flexibility. The articulation on the patella shifts to the superior portion which is thinner and allows sinkage between the condyles. But it also allows less patella flexion during knee flexion and hence further knee flexion for a given quadriceps flexibility (Ritter and Campbell 1987).